The present invention relates to automatic focusing systems for cameras and is particularly directed to a system for automatically focusing a movie camera.
The proper focusing of a camera is critical to the commercial production of motion pictures. Sharpness of focus is an important consideration in the production of realistic, commercially valuable films. Maintenance of a sharply focused image of the principal object of interest in a given scene, particularly when the principal object is in movement toward or from the camera, is a matter of particular challenge in cinematography. In this regard, the concept of depth of field is pertinent. Depth of field is a description of the range of distances an object of interest can be from the camera and remain in acceptably sharp focus without adjustment of the camera lens. Depth of field becomes smaller with increasing lens focal length, with larger lens aperture and with decreasing object distance to the camera. In commercial camera work, whether with film or video cameras, a tendency toward long focal length lenses operated with large aperture puts narrow limits on depth of field in usual practice and, accordingly, on the demands for precise adjustment of lens focus. Under these conditions it is essential that the lens be focused accurately. In the past, this has depended upon the skill of the camera operator or his assistant.
In an effort to overcome these problems, various automatic focusing systems for movie and still cameras have been proposed in the past. These prior art proposals have included ultrasonic ranging systems which have an ultrasonic transmitter for transmitting an ultrasonic acoustic pulse and a receiver for receiving a reflected pulse from the target and producing a signal indicative of the range and utilizing the signal for varying the focus of the lens. Such systems operate satisfactorily under certain situations, such as a single moving target, but present problems when there are multiple targets or when multiple moving targets or objects surround the principle target.
For example, many scenes for motion pictures involve "action" scenes involving several moving or stationary objects in addition to the principle target that may tend to confuse the camera focusing process.
Other systems have utilized light beams such as infrared light beams and the like for the generated signal. These systems suffer from the same problem as the ultrasonic systems in that target discrimination becomes a problem when multiple targets are within the scene of the principle target. One system employed is that of a light system which employs two small mirrors on either side of a prism. The prism reflects light rays to two identical photocells which are connected to a complex electronic comparator circuit. The scene before the camera is viewed by the mirrors, one of which is fixed at a preset angle to the prism and the other which vibrates to scan the scene. A small microprocessor measures the angle of the scanning mirror when the images coincide to calculate the camera-to-subject distance and sends a signal to a focusing motor which adjusts the lens accordingly.
Such autofocusing systems, however, are not fool-proof in that they will always focus upon the nearest dominant object to the camera which may not always be the subject which the operator wishes to focus upon. For this reason, such autofocusing cameras are typically provided with a manual override so that the camera operator can deal with this situation. Thus, these systems are still dependent upon the skill of the cameraman to maintain the scene in focus.
Movie production becomes enormously expensive because of the necessity of retakes for various reasons, including the problem of incorrect focusing. This adds to the already tremendous costs of retakes and the like frequently required during movie production.
It is therefore desirable that a system be available which is selectively and highly discriminating as to targets for movie camera focusing.
The present system employs means for more particularly discriminating selected objects or targets within a scene to be photographed.